Machine to Machine (M2M) Communication is currently an important area for research in fifth generation mobile communication technologies (5G), and is an important field for application for the future of wireless communications. Within the scope of M2M research are NarrowBand-Internet of Things (NB-IoT) subtopics aimed toward terminal features such as low cost, low power consumption, low mobility, and low throughput; that is, to provide within the 200 khz frequency range a low-throughput wireless communication service for NB-IoT low-cost terminals.
In order to reduce signaling overhead and NB-IoT terminal power consumption, the existing research has led to two modes of small data transmission:
1. The control plane optimization mode (CP mode); Data Radio Bearers (DRB) will not be established between the terminal and network, neither will the Access Stratum (AS) security context be established. Data is packaged into Non-Access Stratum Protocol Data Units (NAS PDU) and attached to the control plane signal for transmission; this mode yields relatively obvious conservation of signaling, but the length of data packets transmitted at one time is limited. Relatively larger data packets must be separated into multiple smaller packets for transmission. When coverage is relatively poor, it is easy to lose packets, which makes it difficult for the receiving party to receive a complete data packet.
2. The user plane optimization mode (UP mode): the terminal and the network establish a complete connection, using DRB to transmit data; after data transmission is complete, the bearer information context, the AS security context, etc., is saved in the terminal and network-side through the suspend-activate procedure. When data is transmitted once more, both parties resume the previously stored context through restoring the procedure, and continue to use DRB bearers to transmit data. This mode does not differ greatly from the existing process, and does not limit the length of data packets in any obvious way, but its ability to conserve signaling has limitations.
The existing research has also introduced a process of capacity negotiation between the terminal and network. When the terminal initially attaches, the Attach Request carries an optional Preferred Network Behavior Indication message, indicating the terminal's supportable Network Behavior: 1, whether the control plane optimization mode is supported; 2, whether the user plane optimization mode is supported; 3, recommendation for using the control plane optimization mode or the user plane optimization mode; 4, whether the S1-u data transmission function is supported; 5, whether to require that the short message not be implemented with the attachment.
The MME network element of the network-side will include the Supported Network Behavior in the Attach Accept message to indicate the Network Behavior it is able to accept.
After the terminal and network-side negotiate to adopt the UP mode, when the terminal has uplink data requiring transmission, the air interface and network-side bearer resumption process must be triggered. When the network-side has downlink data requiring transmission, it will first page the terminal and then trigger the terminal to initiate the air interface and network-side bearer resumption process.
In the process of researching the standard RRC connection resumption process of existing technologies, the following approaches have been discussed with regard to the air interface resumption process: 1. defining a new RRC connection resumption request/resumption response message; 2. continuing to use the existing RRC connection establishment process, and adding new instructions; 3. continuing to use the existing RRC connection reestablishment process, and adding new instructions.
Beyond this, some researchers suggest that if the terminal has data it wants to transmit, it is also possible to package the data as NAS PDU or DRB multiplexing, etc., that is carried in some uplink air interface message of the resumption process; for example, carrying it in the air interface resumption request, the air interface connection establishment request, or the air interface connection establishment completion message.
Through the course of our research, we have found that within the foregoing approaches exist the following problems:
1. Presently, after suspension and before resumption, the terminal exists in an Idle state. After resumption the terminal exists in a Connected state, the same state as the terminal before and after the RRC connection establishment process, but the resumption process can be a failure for any number of reasons, such as rejection caused by congestion, wireless link failure caused by anomalies, etc; most of the failures that terminals initiating RRC connection establishment processes can encounter are similar. In addition, the expected processing mode after resumption failure, such as notifying the higher-layer to implement anomaly processing, or waiting a period of time before re-triggering access, etc., are similar to the processing modes used by the terminal after failure to initiate the RRC connection establishment process: if a new RRC connection resumption request message and its corresponding procedure were to be introduced, a large number of processing repetitions would occur.
2. In the resumption process it is possible to transmit NAS PDU of packaged user data; currently, in NAS (non-access stratum) and AS (access stratum) interaction, NAS signaling is only able to trigger the RRC establishment process; if a new resumption request message and its corresponding procedure were to be introduced, the NAS protocol could be more greatly affected. Requiring consideration is how NAS signaling could trigger a new RRC connection resumption request message and its corresponding procedure.
3. Before the terminal triggers the resumption process, various contextual information required by the user plane optimization mode has already been stored on preset terminals, base stations, and related core-network network elements. When a terminal appears because of some anomalous factor, the context information stored in any node on the core-network network element and base station is deficient; this is a new anomaly requiring a fast-paced processing mechanism.
4. Before the terminal triggers the resumption process, various context information required by the user plane optimization mode has already been stored on preset terminals, base stations, and related core-network network elements. How to identify this context information so that the terminal and the base station are able to adopt consistent identity searches and resume the correct context information is as of yet inconclusive. Presently, some terminal identities defined in the standard are primarily allocated in the terminal connection establishment process or used after terminal connection establishment completion, and are not necessarily suitable for identifying the context belonging to the terminal after the connection release. It is thus that an optimized resumption identity definition must be considered.
As for the complicated nature of the connection processing between the terminal and network in the related technology, there has presently yet to emerge an effective resolution.